1. Field of Invention
The present invention relates to a non-planar ring resonator for preventing excitation of backward wave in a passive traveling wave optical resonator.
2. Description of Related Art
Many optical applications require the use of a high-finesse traveling wave optical resonator (a “ring” cavity, so-called because the optical beam travels a one-way around a circuit in the resonator). In particular, cavity-enhanced spectroscopy (“CEAS”) such as cavity ring-down spectroscopy (“CRDS”), optical frequency filters, and optical spatial mode filters, all use high-finesse traveling wave optical resonators in some of their implementations. No real optical element is perfect, however, and the mirrors used for such cavities scatter some of the radiation out of the intended traveling wave. Rayleigh scattering by gas in the optical beam may also occur. Since simple traveling wave resonators are symmetric with respect to direction of optical wave travel, this scattering may excite the backward traveling wave since the optical frequency is identical for forward and backward waves. Unintended excitation of this backward traveling wave often has deleterious consequences.
One of the main stated advantages of a traveling wave cavity is that it ideally does not return optical feedback to the source of optical excitation, such as a laser. Lasers are usually sensitive to such optical feedback and great care is necessary to minimize feedback. The backward wave in the traveling wave resonator will return optical feedback to the laser through the resonator input coupling. Another deleterious effect is the coherent transfer of forward wave energy to the backward wave in CRDS. This transfer of energy during the ring-down event causes the forward wave optical signal, ideally purely exponential in time, to be significantly non-exponential. This effect makes determining the actual optical loss of the resonator difficult and uncertain, thus degrading the performance of the optical spectroscopy.
Although an approximate mitigation to the non-exponential effect is to measure the sum of the forward and backward optical signals, which is approximately exponential if the coupling between forward and backward waves is weak, this solution still requires that the forward and backward beams are detected and summed with equal weight (see, e.g., He et al., Pub. No. US 2012/0242997 A1). This in turn requires at least careful optical alignment, sometimes also precise electronic gain adjustment, and maintenance of that alignment and gain.